AMDGPU: Fix promote alloca pass creating huge arrays

This was assuming it could use all memory before, which is
a bad decision because it restricts occupancy.

By default, only try to use enough space that could reduce
occupancy to 7, an arbitrarily chosen limit.

Based on the exist LDS usage, try to round up to the limit
in the current tier instead of further hurting occupancy.
This isn't ideal, because it doesn't accurately know how much
space is going to be used for alignment padding.

llvm-svn: 269708